The uptake of 5-hydroxytryptamine (5-HT) takes place by a specific Na ion and K ion dependent carrier-mediated process and it is metabolized in the isolated cerebral microvessels. 5-hydroxyindole-3-acetic acid (5-HIAA) was found to be the main metabolite extractable from the microvessels and the incubating media. Anoxia markedly reduced the metabolic rate of capillary 5-HT.